The present invention relates to a hydrostatic bearing apparatus for supporting a movable member such as rotary table or reciprocally movable table through a hydrostatic bearing particularly for the purpose of maintaining constant a gap of the hydrostatic bearing and also relates to a method of controlling the hydrostatic bearing apparatus for achieving the same purpose.
There has been known a hydrostatic bearing in which a gap of the bearing is changed in response to a load change or temperature change and which supports a load by changing a pressure in a static pressure pocket of the bearing. When a hydrostatic bearing of the character described above is applied to, for example, a sliding surface of a machine tool, the change of the gap of the bearing caused by the change of the load directly affects on the performance of the machine tool, and in an adverse case, metallic contact is locally caused, thus damaging the sliding surface. Accordingly, there has been desired to provide a hydrostatic bearing in which the rigidity of the hydrostatic bearing is increased so as not to substantially change the bearing gap even if the load be changed.
In order to make large the rigidity of the hydrostatic bearing, a flow rate regulating valve is interposed between the hydrostatic bearing and an operation oil source to thereby adjust a supply amount of the operation oil in accordance with the load change of the hydrostatic bearing (for example, as disclosed in the Japanese Utility Model Laid-open (KOKAI) Publication No. 50-119027). In the flow rate regulating valve disclosed, an oil passage is formed in a spool slidably accommodated in a valve body for regulating the flow rate and the pressure. This oil passage is formed by drilling working or electric discharging working effected to the central portion of the spool.
Furthermore, in the conventional technology, for example, there are provided control methods of controlling an oil temperature of an operation oil utilized for the hydrostatic bearing of a machine tool, and the methods includes a method in which the temperature of the operation oil in an operation oil tank is accorded with a room temperature or a temperature of the machine tool by utilizing a refrigerating machine or cooler, and a method in which the operation oil temperature is accorded with a fixed value. Moreover, as a control method of a pressure of the hydrostatic bearing, a supply pressure of a hydraulic pump is made constant or a constant pressure valve is interposed on the way of the operation oil supply passage.
However, in the conventional technology described above, when drilling or boring working is effected to the inside of the spool to form an oil passage by the drilled bore or hole having a long length, it is considerably difficult to effect a fine drilling working with a large ratio of the length to the diameter of the hole and also difficult to ensure a size of the diameter, which results in a difference between the desired theory and the actual working. Furthermore, since the oil passage is formed with a small diameter hole, it is difficult to confirm the presence of the hole from the outside.
Furthermore, in the conventional apparatus, a good pressure-flow rate characteristic of the flow rate regulating valve and a good linearity thereof have not been achieved, and thus, it is difficult to achieve an initial object.
Further, it is considered to form a bearded groove on an outer peripheral surface of the spool instead of the hole, i.e. oil passage, but the bearded groove has not formed linearly, so that the working of such bearded groove requires troublesome working, and even if the working shape and dimension be made constant, it is difficult to stably produce the hydrostatic bearing always with the performance obtained by tests being maintained.
Moreover, in the oil temperature controlling method described above, it is aimed to coincide the temperature of the operation oil with a fixed temperature such as room temperature or a temperature of the machine. Accordingly, it is impossible to maintain the gap of the hydrostatic bearing because of the change of a viscosity of the operation oil due to a heat generation of the operation oil in the static pressure pocket of the hydrostatic bearing by a shearing force in accordance with, for example, the increasing or decreasing of a revolution number of a rotary table of a vertical lathe or reciprocation number of movement of a table of a planer, or the change of the pressure of the operation oil in the static pressure pocket in accordance with the change of the viscosity of the operation oil or an external force such as load. For the reasons described above, in a machine tool in which high performance, high quality and speed-up operation are required, the working performance of products is badly affected by the change of the gap of the hydrostatic bearing.